Since 1978, a longitudinal study -- the Ottawa Prenatal Prospective Study (OPPS) -- has been investigating a low risk population, the effects of marihuana and cigarettes used during pregnancy. The general objectives of this competitive renewal are twofold and take advantage of the age of the OPPS subjects (20-23 years) who have been assessed since birth. We will continue to assess the young adults focusing upon the domains impacted by either prenatal marihuana or cigarettes at younger ages and examine the predictive validity of the infant and early childhood tests in terms of the continuity of effects over two decades. The second thrust of the proposed work will be to examine parameters related to regular marihuana use by the OPPS offspring, the evaluation of such use upon cognitive performance and an investigation of possible effects after prolonged cessation of regular usage. We have identified, via self-report and urinalysis, that approximately 30 percent of the OPPS 16 1/2 to 19 year old subjects are using marihuana on at least a weekly basis and have been doing so for a year or more. Approximately 20 percent in this age range have used marihuana to that extent in the past but have quit for at least six months. Among these past and present users over half averaged at least 5 joints per week. By using both within and between subject comparisons we will address such question as: whether there are predrug cognitive differences between non-users and future users; whether there is an impact upon cognitive performance that persists beyond the acute intoxication phase either while there may be a drug residue lingering in the CNS or well after such a residue has been cleared. We have the unique opportunity to examine, for the first time within the same subjects, cognitive performance before the initiation of regular marihuana use, during such use and after prolonged cessation. The low-risk nature of the OPPS sample, the vast array of background information, and the longitudinal neuropsychological data spanning infancy to young permits an examination of both the etiology of marihuana use and the consequences on cognitive performance in a manner identified by many researchers as being the pragmatically optimal protocol.